Schiehallion experiment

Long ago, scientists wanted to know about Earth.

They chose a big mountain in Scotland. It was shaped like a neat cone.

A heavy tool hangs on a string. The mountain's pull moves the tool a tiny bit. This pull shows how much the mountain weighs. Scientists used this to learn about Earth's weight. They also learned about the Sun and other planets. It was a very smart way to learn!

In 1774, scientists wanted to learn about the Earth.

They wanted to find the Earth's mean density. Density is how much matter is packed into a space. To do this, they went to a mountain in Scotland. The mountain is named Schiehallion.

It has a very even shape. This made it a good place for their work.

They used a tool called a plumb-line. This is a heavy weight on a string. Normally, gravity pulls the weight straight down. But the mountain has its own gravity too. This pull tugs the weight slightly to the side. This tiny shift is called deflection.

Scientists measured this shift from both sides of the mountain.

They also had to find the mountain's volume. Volume is the amount of space the mountain takes up. A man named Charles Hutton used contour lines to help. These are lines on a map that show height. By knowing the mountain's size and pull, they could guess the Earth's density. This helped them learn about the Sun and other planets, too.

Scientists once wanted to know how heavy the Earth really is. They wanted to find its mean density, which is how much matter is packed into a space.

Knowing this would help them understand the Sun and other planets. Before this, they only knew how planets compared to each other. By finding the Earth's density, they could find the actual mass of the whole solar system. This was a huge goal for science in the 1700s.

To do this, they used a clever way to measure gravity. They used a plumb-line, which is a heavy weight hanging from a string.

Usually, gravity pulls this weight straight toward the center of the Earth. However, a huge mountain nearby has its own gravity too. This pull tugs the weight slightly to the side. This tiny shift is called deflection. Scientists measured this angle from both sides of the mountain. They then compared the mountain's volume to this pull.

This idea was not new, but it was very hard to do. Isaac Newton thought about this experiment long ago. He was a very famous scientist who wrote about gravity. However, Newton was pessimistic about the idea. He thought the pull from a mountain would be too small to measure. He believed gravitational effects only showed up on a planetary scale. But a team of scientists proved him wrong in the 1770s.

The famous Schiehallion experiment happened in the summer of 1774.

It took place around Schiehallion, a mountain in Perthshire, Scotland. Nevil Maskelyne, the Astronomer Royal, led the team. He was helped by mathematicians like Charles Hutton and Reuben Burrow. They used special tools like a brass quadrant and a precision pendulum clock. They even built small observatories on the mountainside. The work was hard because of the rain and mist.

This experiment helped prove how gravity works everywhere. Maskelyne found that the Earth's density was about double the mountain's density. This confirmed Newton's laws of gravity were correct.

To map the mountain, Charles Hutton used contour lines. These are lines on a map that show height. This was a new way to show the shape of the land. His method is still used by mapmakers today. The experiment turned a big mystery into a known fact.

The Schiehallion experiment was a landmark scientific study from the 18th century. Its primary goal was to determine the mean density of the Earth. Mean density describes how much mass is packed into a specific volume.

Before this experiment, scientists only knew the relative ratios of masses in our solar system. They knew how much heavier one planet was than another, but not their actual values. By calculating the Earth's density, they could finally estimate the true masses of the Sun, the Moon, and all the other planets. This was a massive step in understanding the scale of the universe.

The experiment relied on a phenomenon called gravitational deflection. Normally, a plumb-line—a weight hanging from a string—points directly toward the center of the Earth. However, a large mass like a mountain exerts its own gravitational pull. This pull tugs the plumb-line slightly away from the true vertical.

To measure this, scientists compared the plumb-line's position to the position of a known star. By measuring this tiny angle of deflection on opposite sides of a mountain, they could calculate the mountain's mass. If they also knew the mountain's volume and its rock density, they could use math to find the Earth's density.

Finding the right location was a major challenge. Earlier, in 1738, French astronomers Pierre Bouguer and Charles Marie de La Condamine tried this on Mount Chimborazo in Ecuador.

They detected a tiny deflection of 8 seconds of arc, but the difficult terrain and climate made it hard to be certain. Later, during the survey of the Mason–Dixon line, Henry Cavendish noticed errors in surveying tools. He realized the Allegheny Mountains were likely pulling on the instruments. This prompted Nevil Maskelyne, the Astronomer Royal, to propose a more controlled experiment in Scotland.

Schiehallion was chosen as the ideal site in 1774.

It was located in Perthshire, Scotland, and stood isolated from other large hills. This isolation ensured that other mountains would not interfere with the measurements. The mountain also had a very symmetrical east-west ridge. This shape made the complex mathematical calculations much simpler. Its steep slopes allowed the team to set up stations close to the mountain's center of mass to maximize the effect.

The scientific team was exceptionally well-equipped for the task. Nevil Maskelyne led the project, supported by mathematicians Charles Hutton and Reuben Burrow. They used high-precision tools, including a brass quadrant from a previous expedition and a regulator, which is a very accurate pendulum clock. They even built small observatories and a bothy for equipment on the mountainside. The work was physically demanding and often interrupted by mist and rain. Maskelyne conducted hundreds of observations on various stars to ensure his data was accurate.

One of the most important results came from the surveying work. To find the mountain's volume, Charles Hutton had to divide it into many vertical prisms. He processed thousands of bearing angles from over a thousand different points. To manage this massive amount of data, Hutton developed a method of marking points of equal height.

These became known as contour lines. This technique allowed him to visualize the terrain and became a standard tool in modern cartography. This helped turn a chaotic mountain shape into a precise mathematical model.

In the end, the experiment was a profound success. Maskelyne announced that the Earth's mean density was approximately double that of Schiehallion.

This result provided powerful evidence for Isaac Newton's law of universal gravitation. Although Newton had been skeptical that such a small deflection could be measured, the experiment proved him wrong. It confirmed that gravitational effects are detectable even on a local scale. The success of the project earned Maskelyne the Copley Medal and helped solidify the Newtonian system of physics in the scientific world.